1. Field of the Invention
The present invention relates to a semiconductor device, more particularly to a high-breakdown-voltage semiconductor device such as a MOSFET for power control, whose material is silicon carbide (SiC).
2. Description of the Related Art
As a power semiconductor device material of the next generation, SiC is expected to be used. As compared with Si, SiC has excellent physical properties in that the band gap is 3-fold, the intensity of breakdown electric field is about 10-fold, and the thermal conductivity is about 3-fold. When these properties are utilized, a power semiconductor device capable of operating at a high temperature with an ultra-lowloss can be realized.
There exist various types of high-breakdown-voltage semiconductor devices utilizing such SiC properties, but, for example, a double-implantation-MOSFET (DIMOSFET) is known in which a p-well and a source region are formed by ion implantation (see R. Kosugi et al., Materials Science Forum Vols. 457 to 460, pp. 1397-1400 [2004]). The DIMOSFET is easily manufactured, because a planar process is used in which a channel is formed by an ion implantation process with a satisfactory precision. Moreover, since gate driving is controlled by voltage, a power of a driving circuit can be reduced, and this element is a superior element suitable for a parallel operation.
However, the DIMOSFET has the following problem. An n-type source region of an SiC-DIMOSFET is usually formed, when phosphorus that easily lowers resistance is ion-implanted with a high dosage, and thereafter an activating thermal treatment is performed at about 1600° C. In this case, since phosphorus having a mass number of 31 and being comparatively heavy is ion-implanted on high dosage conditions, and the thermal treatment is performed at a high temperature of around 1600° C., an ion-implanted region of the SiC surface is badly damaged. As a result, a preferential sublimation phenomenon of Si occurs from the implanted region. Therefore, surface roughness of 10 nm or more is generated in the source region. Thereafter, when a gate insulating film is formed in such a manner as to range from the source region to a p-type base area by a thermal oxidation process, a CVD process or the like, the surface roughness of the source region is reflected as such also in the gate insulating film on the source region. As a result, electric long-term reliability of the gate insulating film is remarkably impaired.
To solve the above-described problem, there is reported a double epitaxial MOSFET (DEMOSFET) in which the p-type base area (well) is formed by an epitaxially grown film only (S. Harada et al., IEEE Electron Device Lett. 25, pp. 292-294 [2004]).
However, in the above-described method, the manufacturing process includes epitaxial growth requiring a long time. Therefore, there has been a demand for realization of a high-breakdown-voltage semiconductor device which obtains excellent performance having an ultra low on-resistance by use of an ion implantation technology capable of reducing step time, and SiC original physical properties and which can largely improve the long-term durability of the gate insulating film.